Pretty simple really. Port numbers are effectively topic identifiers. The packet payload contains the data. Both producers and consumers run on a fire and forget basis. i.e. avoid any RPC patterns.

Obviously, my designs are for systems that work well with those principles. It is by far not a generalized architecture. I still use MQTT, TCP, WebSockets, IPC, UDS etc where they provide essential and critical capabilities.

My philosophy is simple. Use the simplest, easiest tool that gets the job done without needless complexity or difficulties. If you are having too many problems, then revise the design.